1. Field of the Invention
This invention relates generally to improvements in control apparatus and, more particularly, but not by way of limitation, to a control apparatus for use with a drawworks assembly having a work piece suspended from and applying tension to, a cable, one end of the cable being wound on a drum, the rotation of which is controlled by a power brake mechanism.
2. The Description of the Prior Art
Various types of control apparatus have been developed for use with a drawworks assembly having a work piece suspended from, and applying tension to, a cable, one end of the cable being wound on a drum, the rotation of which is controlled by a power brake mechanism. Generally, such control apparatus senses the tension in the cable and attempts to control the payout of cable from the drum when the tension in the cable exceeds a predetermined level. Such control apparatus have been generally unacceptable for any one of a number of the following reasons: Many such control apparatus require extensive modification to the existing drawworks; many such control apparatus are capable of only gross control of cable payout due to a general inability to precisely control the power brake mechanism; many of such control apparatus require a feedback mechanism to sense the rotation of the drum in order to prevent drum runaway; and many of such control apparatus depend upon an electrical power source and thus are highly susceptible to the introduction of spurious noise with a subsequent degradation in performance.
For example, the automatic well drilling apparatus described in U.S. Pat. No. Re. 24,818 issued Apr. 26, 1960 to W. C. Gresham et al., is subject to several of the above described disadvantages. The control apparatus described therein is powered electrically and thus is subject to the introduction of extraneous noise and thus must either be shielded or suffer a degradation in performance under certain field operating conditions. In addition, the control apparatus exhibits a marked degree of inflexibility since it is unable to actuate the power brake mechanism at any point other than a full brake release or a full brake engage. When the Gresham control apparatus is used on a drawworks assembly whose braking system has been worn to the point where lobes have been formed on the brake-engaging surfaces of the drum, the performance of this system becomes particularly irratic and disruptive.
The control apparatus shown and described in U.S. Pat. No. 3,759,489, issued to Harry C. Jones on Sept. 18, 1973, exhibits many of the disadvantages described above in relation to the Gresham control apparatus. For example, the Jones apparatus being electrically powered, is also susceptible to the introduction of spurious noise with the resulting degradation in performance. In addition, the Jones control apparatus is similarly unable to manipulate the power brake mechanism in other than a gross on-off manner. However, the Jones control apparatus also displays the additional disadvantage of requiring a drum rotation feedback sensor in an attempt to prevent drum runaway.